We have an API gateway that has ran wonderfully for many years and we send email from the mainframe through it. In the past couple of years, occasionally we have an account that will not process, we get an error 'unknown destination in address string'. If I put the fully qualified domain and post office in the address, it will process fine.

This only happens on a couple of addresses, the other addresses in that domain/po process fine. What is missing that the domain cannot process that record without the domain/post office information?